An encrypted ID takes as its initial value a tag ID that constitutes an original ID of a wireless tag, and is encrypted using an encryption key each time an ID inquiry is received from a reader/writer. Another encrypted ID also takes the tag ID as its initial value, and is encrypted using an encryption key each time an ID inquiry is received. The wireless tag gives confidential variability to the ID by transmitting a combination of the encrypted ID and the another encrypted ID in reply. The reader/writer can obtain the original ID merely by decoding the two encrypted values until they match, and the decoding load does not increase even if the number of wireless tags that are reading targets increases.